An insulated electrical wire may be used in equipment such as heating equipment or in a fire alarm device for which safety under a high temperature is required. Further, an insulated wire of this type is also used in the environment of an automobile, which is heated to a high temperature by the engine. An insulated wire formed by an electrical conductor which is coated with heat resistant organic resin such as polyimide, fluorocarbon resin or the like has generally been used for the above purposes.
Mere organic coatings are insufficient for applications requiring a high heat resistance or for use in a environment for which a high degree of vacuum is required, because an organic coating has an insufficient heat resistance, and due to a gas emission property and the like. Thus, an insulated wire having a conductor inserted in an insulator tube of ceramics, or an MI cable (Mineral Insulated Cable) having a conductor inserted in a heat resistant alloy tube of a stainless steel alloy etc. which is filled with metal oxide powder of magnesium oxide etc., or the like has been used in high temperature and vacuum environments.
A fiber-glass braided insulated wire employing textile glass fiber as an insulating member etc. is listed as an insulated wire satisfying flexibility and heat resistance requirements.
In the aforementioned insulated wire coated with a heat-resistant organic resin, the highest temperature at which an adequate electric insulation can be maintained, is about 200.degree. C. at the most. Therefore, it has been impossible to use such an organic insulation coated wire under conditions requiring a guarantee of an adequate electrical insulation at a high temperature of at least 200.degree. C.
Further, the insulated wire which is improved in its heat resistance by an insulator tube of ceramics, has disadvantages such as an inferior flexibility. The MI cable comprising a heat resistant alloy tube surrounding a conductor, has an increased outer diameter with respect to the conductor radius. Thus, the MI cable has a relatively large cross-section with respect to electric energy that can be carried by the conductor passing through the heat resistant alloy tube. In order to use the MI cable as a wire for winding a coil in a bobbin or the like, however, it is necessary to bend the heat resistant alloy tube in a prescribed curvature which is difficult. For example, it is difficult to obtain a suitable winding density since the tube forming the outer enclosure is thick compared to the conductor.
Further, when the fiber-glass braided, heated resistant, insulated wire is employed and worked into a prescribed configuration as required for its application, the network of the braid is disturbed resulting in a breakdown. In addition, dust of glass is generated by the glass fibers. This glass dust may serve as a gas adsorption source. Therefore, when the fiber-glass braided insulated wire is used in an environment for which a high degree of vacuum is required, it has been impossible to maintain a high degree of vacuum due to the gas adsorption source by the glass dust.